1. Field of the Invention
The present invention generally relates to a motor driving apparatus including an H-bridge circuit constituted by a plurality of transistors some of which are driven under the control of a pulse-width modulated signal of which duty ratio is controllable (hereinafter, such driving will be referred to as the driving under PWM control or PWM-driving or the like). More specifically, the invention is concerned with a motor driving apparatus which is capable of continuing or stopping selectively and effectively operation of an electric motor employed as a drive source in dependence on abnormal conditions prevailing upon occurrence of a short-circuit fault in a transistor circuit. The invention can profitably find application to a motor-driven power steering system of a motor vehicle, being understood, however, that the invention is never restricted thereto.
2. Description of Related Art
For having better understanding of the invention, description will first be directed to the technical background thereof. FIG. 4 is a block diagram showing generally a circuit configuration of a conventional motor driving apparatus known heretofore. In this case, it is assumed that the motor driving apparatus is adapted for driving an electric motor employed for generating an assist torque in a motor-driven power steering system of an automobile or motor vehicle for assisting a driver in manipulating a steering wheel. A typical example of such motor driving apparatus is disclosed, for example, in Japanese Unexamined Patent Application Publication No. 158455/1990 (JP-A-2-158455).
Referring to FIG. 4, an electric motor 2 whose operation is to be controlled is connected between output terminals 1a and 1b of an H-bridge circuit 1 which in turn is constituted by a plurality of transistors such as FETs (field-effect transistor). In the illustrated circuit, there are employed four transistors or FETs Q1 to Q4, wherein a diode is connected in parallel to each of the transistors Q1 to Q4 in order to allow a motor current IM generated upon stopping of the electric motor 2 to flow through the diodes.
The H-bridge circuit 1 has one end 1c connected to a power supply source 3 which may be constituted, for example, by an onboard battery of a motor vehicle, wherein a relay 4 is inserted between the one end 1c of the H-bridge circuit 1 and the power supply source 3 for the purpose of controlling (i.e., enabling or disabling) the power supply to the electric motor 2. Further, the H-bridge circuit 1 has the other end 1d connected to the ground potential GND, wherein a shunt resistor 5 is inserted between the other end 1d and the ground GND for detecting an electric current I flowing to or through the H-bridge circuit 1.
There are provided a plurality of driving circuits 6 to 9 in correspondence to the transistors (FETs) Q1 to Q4, respectively, wherein PWM signals (or on-signals, i.e., signal for operating the transistor in the constantly or continuously conducting state which may also be referred to as the on state) P1 to P4 are selectively applied to the individual transistors Q1 to Q4.
A voltage detecting circuit 11 is provided for detecting a relay contact voltage VR applied to the one end 1c of the H-bridge circuit 1, while a current detecting circuit 12 is provided for detecting a current value Id on the basis of a voltage appearing at one end of the shunt resistor 5. Further, a torque sensor 30 is provided for detecting a steering torque T applied to a steering system by a driver through a steering wheel in the assumed case where the electric motor 2 is employed for generating an steering assist torque, as mentioned previously.
A control means 20 implemented by using a microcomputer fetches the detected current value Id, the relay contact voltage VR and the output of the torque sensor 30 indicative of the applied steering torque T to thereby output a stop signal S for on/off control of the relay 4 as well as drive control command signals C1 to C4 for controlling the driving circuits 6 to 9, respectively.
Next, referring to FIG. 4, description will be made of operation of the conventional motor driving apparatus on the assumption that the motor driving apparatus is applied to a motor-driven power steering system of a motor vehicle for generating an assist torque for assisting a driver in manipulation of a steering wheel.
For generating a desired assist torque in dependence on the applied steering torque T as detected, the control means 20 first determines selectively the transistors (e.g. transistors Q1 and Q4) to be driven under the PWM control on one hand and to be driven in the on-state on the other hand and at the same time determines a desired duty ratio value of a PWM signal (P1) for the transistor selected for the PWM-driving (e.g. Q1).
More specifically, one (e.g. transistor Q2) of the transistors Q1 and Q2 connected to the power supply source 3 and one (e.g. transistor Q3) of the transistors Q3 and Q4 connected to the ground GND are turned off. In this state, the other one (e.g. transistor Q1) of the transistors Q1 and Q2 connected to the power supply source 3 is driven under the PWM control with the other one (e.g. transistor Q4) of the transistors Q3 and Q4 connected to the ground GND is driven continuously in the on-state.
Thus, the electric motor 2 connected between the output terminals 1a and 1b of the H-bridge circuit 1 is driven for an angular distance in a direction under supply of a motor current IM as commanded by the control means 20.
In that case, in order to enhance the controllability of the duty ratio of each of transistors Q1 to Q4 while suppressing generation of heat and noise upon turning on/off of the transistors, only the transistor Q1 connected to the power supply source 3 is driven under the PWM control with the transistor Q4 connected to the ground GND being maintained steadily in the conducting state (i.e., on state).
Parenthetically, when the electric motor 2 is to be driven in the reverse direction, then the operating states of the transistors are so reversed that the transistors Q2 and Q3 are driven with only the transistor Q2 being driven under the PWM control while the transistors Q1 and Q4 are both turned off.
At this juncture, it is noted that when a short-circuit fault occurs in any one of the transistors Q1 and Q4 or Q2 and Q3 during operation of the electric motor 2, the electric current I flowing through the H-bridge circuit 1 or the motor current IM becomes excessively large. To say in another way, an overcurrent flows through the H-bridge circuit 1 and hence through the electric motor 2.
Such overcurrent is then detected by the current detecting circuit 12, and the detected current value Id indicating the overcurrent is supplied to the control means 20 from the current detecting circuit 12. In response, the control means 20 generates immediately a stop signal S for interrupting application of the PWM signal to the H-bridge circuit 1 to thereby stop operation of the electric motor 2 in order to protect the H-bridge circuit 1, the electric motor 2, the power supply source 3, the relay 4, a fuse (not shown) and so forth from injury or damage due to the overcurrent.
However, when the motor driving apparatus is employed in association with the motor-driven power steering system of a motor vehicle with the electric motor 2 generating an assist torque for the motor-driven power steering system, the stop processing executed by the control means 20 as mentioned above means that the assist torque applied to the steering wheel of the motor vehicle is abruptly reduced to zero during operation of the motor vehicle, which will give rise to not a little shock and uneasiness to the driver or operator. For this reason, the processing for generating the stop signal S immediately in response to occurrence of the short-circuit fault is not preferred.
As is apparent from the above description, the motor driving apparatus known heretofore suffers a problem that because the electric motor 2 is stopped abruptly by interrupting immediately the power supply to the electric motor 2 when the detected current value Id indicates abnormality during operation of the electric motor 2, inconveniences such as mentioned above may arise, although it depends on the applications for which the electric motor 2 is employed.